An integrating sphere is an optical component that is formed of a hollow spherical or hemispherical cavity with its interior covered with a diffusely reflecting coating. The integrating sphere normally has holes on the sphere wall for measurement and for illumination of the sphere and surface. Light rays incident on any point on the inner surface are scattered via diffuse reflection. After many reflections the illumination on, and radiance from, all parts of the sphere wall is highly homogeneous. When an integrating sphere is used to measure a hemispherical reflectance of a surface, the surface under test (SUT) may be situated at the sample port, at an aperture on the sphere, or inside the sphere such that the surface is illuminated from all directions inside the sphere. The measurement of the light emitted or reflected from the surface is conventionally done through a separate measurement hole through the side of the sphere.
One application for the integrating sphere is to simulate real-world illumination conditions upon displays of devices such as cell phones, tablets or televisions in order to evaluate their contrast ratios; see for example a publication of International Committee for Display Methodology (IDSM), entitled “Information Display Measurements Standard”, version 1.03, Jun. 1, 2012, which is incorporated herein by reference. To make such a hemispherical reflection condition without the specular component, an additional hole is provided through the sphere which aligns to the position where the specular component of the reflection originates, as illustrated for example in FIG. 1, which reproduces FIG. 1 of Section 11.3.2, pg. 204, of the above cited publication. The location of the specular hole is exactly mirrored from the position where the measurement is taken, both with respect to the normal direction of the sample under test. Usually this specular port is directed into a dark room or a light trap that is external to the integrating sphere. To measure diffuse reflectance including the specular component, a filler plug must be used in place of the light trap.
This design has several limitations, including the following: a) only fixed angles of diffuse reflectance with the specular component excluded can be measured, b) the reflectance of the filler plugs may not be uniformly matched to the sphere wall and c) the radiance for highly mirror-like surfaces may be less accurately measured when the specular region is aligned to this non-uniform filler plug.
An objective of the present invention is to provide an improved integrating sphere type device that addresses at least some of the aforementioned limitations of conventional integrating sphere devices.